Biological conditions are most often monitored in clinical settings. While effective in many situations, clinical tests will often fail to detect rare or transient events. Daily wearable sensors and systems are needed for ambulatory data collection over extended periods of time.
Even clinical testing systems have drawbacks. Biological testing, such as during stress testing, is most often conducted clinically with ECG (electrocardiography) or EEG (electroencephalography) measurements. The ECG systems record heart activity. EEG systems record brain activity. Both systems detect electrical signals from multiple electrodes that are adhered to the skin for data collection. Typical electrodes are wet electrodes, which are employed to attempt to reduce interference from impedance with the skin to electrode interface. However, the wet electrodes cause skin irritation and discomfort when conductive gel or paste is adhered to skin for low-impedance electrical connections. In addition, a large number of electrodes at discrete locations are used to obtain independent signals. The many-wire architecture from body to instrument can take extensive time for preparation, can constraint normal body motion, and electrodes are often loosened during testing and fall off.
This has been a long recognized problem, but the ECG and EEG systems with wet contact electrodes remain state of the art. Prior efforts to improve the ECG and EEG systems have focused on non-contact sensing. An early system introduced capacitive coupling to the skin. See, P. C. Richardson, “The insulated electrode: A pasteless electrocardio-graphic technique,” Proc. Annu. Conf. Eng. Med. Biol. (ACEMB'20), vol 9, p. 157 (1967). Research into the non-contact sensing techniques have continued for decades. See, e.g., Farzad Hosseini, Dietmar Schroeder and Wolfgang H. Krautschneider, “Capacitive sensors for Detection of the Movement Artifacts in Active Capacitive Electrocardiography Electrodes”, Biomedical Engineering International Conference (BMEiCON), pp. 1-4 (2012). These non-contact methods don't require a direct connection with skin and allow for electrodes to be in clothing or applied with small adhesive patches. After decades of research in non-contact sensing, the ECG and EEG wet electrodes remain important because the non-contact sensing still suffers from excessive noise due to movement between the subject and sensor and because of triboelectricity. Other sources of noise include environmental interference (caused by active grounding resulting from surface contact) and high frequency noise created by muscles.